First Ion Temperature Measurements in the MAST-U Divertor via Retarding Field Energy Analyzer

This study presents the first ion temperature ($T_i$) measurements from the MAST-U divertor using a Retarding Field Energy Analyzer (RFEA). Embedded within the flat tile of the closed divertor chamber, the RFEA captures $T_i$ profiles across various plasma scenarios, including transitions to the Super-X configuration. Measurements were conducted under steady-state and transient plasma conditions characterized by a plasma current ($I_p$) of 750 kA, electron density ($n_e$) between $2.2 \times 10^{19}$ and $4.45 \times 10^{19}\,\text{m}^{-3}$, and Neutral Beam Injection (NBI) power ranging from 3.0 MW to 3.2 MW. The ion temperatures, peaking at approximately 17 eV in steady state, were compared with electron temperatures ($T_e$) obtained from Langmuir probes (LP) at identical radial positions. The study also examined ion saturation current density ($J_{\text{sat}}$) signals to using methodologies similar to previous MAST experiments. Preliminary findings reveal a $T_i/T_e$ ratio ranging from 1 to 2.2. Additionally, high temporal resolution measurements (100 $\mu s$) captured the dynamics of Edge Localized Modes (ELMs), showing $T_i$ peaks at 16.03 +- 1.84 eV during ELM events, nearly three times higher than inter-ELM temperatures.